When I first started doing the research that led to this site, I found that there were two basic categories for crime and suspense fiction: hard-boiled and noir. But where do you draw the line between them? What characteristics do each have? And (as the Talking Heads might say) how did we get here?
To begin with, hardboiled and noir writing comes out of two distinct traditions. Hardboiled writing evolved from the marriage of classical detectives stories (featuring heroes such as Sherlock Holmes) and American heroic fiction, such as Westerns and the Nick Carter stories. Dashiell Hammett first joined these two types of heroic fiction in his Continental Op short stories, the novels Red Harvest and The Dain Curse, and other novels. Hardboiled fiction can usually be classified as mystery.
Noir fiction originated in social realism. Many of the great writers of the late 1940s and early 50s cited people like Sinclair Lewis and (especially) John O'Hara as big influences. Earlier works by James M. Cain, Horace McCoy, and others similarly uses realistic social backgrounds and situations to drive crime stories. These works are classified as suspense.
A key difference is the treatment of the protagonist. In hardboiled writing, this character can safely be referred to as the hero. While these men and women may face the danger of bodily harm or even death, they can rest easy knowing they're not going to Hell. Though they live in a corrupt world they choose to live by their own codes of honor, doing the right thing no matter what the cost.
The noir protagonist is altogether more ambiguous. In stories such as Cain's The Postman Always Rings Twice he may actually be a murderer. Even when he's on the side of goodness and light he's in constant moral danger. Will he succumb to temptation, whether it be love, power, or a big payday? In noir fiction it's an open question.
Hardboiled and noir fiction also treat the relationship of the protagonist to the other characters differently. In a hardboiled story, he's usually an outsider, coming in to resolve a situation that developed before he was on the scene. This may be a function of his profession (cop, private eye) or he may just be an interested bystander, but he's usually late to arrive. Noir protagonists are usually involved from the beginning. They know the other characters intimately and it's the changing relationships between these characters that generates conflict.
Also, hardboiled characters tend to be detached from the situation they're investigating. They keep themselves a little aloof, a little reserved. While they may get involved in a case on a personal, rather than professional, level, it represents only a part of their life, one that can be shut away once it's over. Noir characters are deeply involved; their entire lives are bound up in the story, their future is determined by the outcome. It's not unusual for the main character to die at the end (or go directly to Hell; see Jim Thompson). It's the difference between being committed and being involved. For example, when you're making ham and eggs, the chicken is involved - but the pig is committed!
What this boils down to is conventions. Hardboiled novels, and private eye novels in particular, are bound by certain rules: the good guy never kills except in self-defense, he doesn't lie unless he has too, he doesn't steal, etc. In noir, all bets are off. The main character may get away with murder or go to the chair; he may wind up with the money or a bullet in the head; he may get the girl or the girl may get him.
So much for the differences. What about the similarities? First, language and style. Hammett and W. R. Burnett (among others) were extremely influential in taking the word on the street and getting it on the printed page. People in these stories talk the way real people do. In addition, these were working-class stories, aimed at and featuring working-class people. Many of these stories were set in a realistic (though not always accurate) criminal underworld; hardboiled fiction just visited, while noir fiction lived there. These stories were concerned with outsiders, people who can't turn to the normal institutions of society such as the police. These people frequently had to appeal to other forms of justice, or dish it out themselves.
Fortunately, you don't have to worry about categories to enjoy a book. Names like Thompson, Raymond Chandler, Ross Macdonald, and David Goodis demand to be read, not pigeonholed. Many writers, like Leigh Brackett and Fredric Brown, wrote at a high level in both fields. In the end there's only one thing to say: